In recent years, dispersions of micron-size bubbles (microbubbles) have become increasingly important for various applications in the fields of chemistry, chemical engineering and microbiological engineering. Improved flotation processes involving fine particulate and oil dispersions are encountered in industrial sanitary and food processing waste waters, and of the fighting of fires.
A microbubble dispersion can consist of very small surfactant-stabilized bubbles, typically formed as a 50% to 65% dispersion of gas in a liquid. The quality of a microbubble dispersion may be defined as the percentage of gas in the gas plus liquid dispersion. Gas bubbles stabilized with a surfactant (soapy) film tend to maintain a small size with or without stirring. Surfactant-stabilized microbubbles also tend to resist coalescence because the surfactant tends to orient at the gas-liquid interface, forming a charged bubble surface that repels other bubbles. The less that a microbubble dispersion coalesces in one minute, the better is its stability. The microbubbles typically have diameters of 15 to 120 microns, but they also may be larger or smaller than these sizes. They often are referred to as colloidal gas aphrons (CGA) to underscore the colloidal properties of these very small bubbles. The word "aphron" has been coined to mean a fluid encapsulated in a thin aqueous shell--a true bubble. A CGA consists of an inner pocket of air surrounded by an aqueous double layer film which is surrounded by the continuous phase. Both the gas interface and the film/continuous phase interface of the film have higher surfactant concentrations than the bulk fluid. This double layer phenomenon stabilizes the CGAs by preventing them from coalescing. First, an electric potential gradient is set up by the orientation of the molecules at the interfaces. CGAs created with the same surfactant will have similar surface charges and will repel each other, preventing contact. Also, the film acts as a slightly springy wall when CGAs come close to each other. The combination of these two effects results in a foam that is stable enough to be pumped, has a very large surface to volume ratio and exhibits a slow rise velocity.
In early studies, CGAs were produced by a venturi device that required large recirculation velocities in order to form a uniform bubble size distribution. See, for example, U.S. Pat. No. 3,900,420 (Sebba). An improved CGA generator was later devised which used a spinning disk bracketed by a pair of baffles to produce 1-2 liters of CGAs per minute. Other devices for microbubble generation also have included close tolerance rotary pumps, packed beds of solid particles including glass spheres, air-sparged hydrocyclones, and a tube reactor using sintered bayonet fingers.
However, the aforementioned spinning disk method to produce microbubble dispersions in small quantities may have limited potential for economical scale-up, and no device to date is believed to provide continuous high volume generation of a fine microbubble dispersion. Low-cost efficient methods and devices for producing high quality dispersions of small microbubbles on a large scale need to be developed to take advantage of many industrial applications where such dispersions could significantly reduce energy consumption and cost in the performance of these applications. For example, enormous electrical costs are incurred in sparging (beating) air into sludge in order for biodegradation to occur in activated sludge processing plants. The present invention provides a novel technique for economically and continuously producing large quantities of high quality microbubble dispersions at the rate of 30 to 50 gallons per minute, and it represents a considerable improvement in generating microbubbles with good stability and small bubble size, minimum void gas and reasonable operating cost.